Cant use Set Utility at the same time

is it normal, that we cant use Set Utility at the same time?

$set('start_date',$date->subMonth()->startOfMonth()->setDay(28));
$set('end_date',$date->startOfMonth()->setDay(20));

$set('start_date',$date->subMonth()->startOfMonth()->setDay(28));
$set('end_date',$date->startOfMonth()->setDay(20));

i tried that code it oly works for the first line, the second line doesnt work